A piezoelectric crystal diaphragm, a thin plate of a piezoelectric crystal, such as quartz, or barium titanate, provided with electrodes on both sides, undergoes changes in thickness when subjected to potential differences between the electrodes--piezoelectric effect--. Such piezoelectric diaphragms generally have high resonance frequencies and have been used, as a device for converting electrical signal energy into sound, only in tweeters, i.e. loudspeaker for handling only the higher audio frequencies, or in other simple sound producers.
The piezoelectric crystal diaphragm has been frequently accommodated in a thin, flat casing with a small circular opening formed in the center of its wall, arranged such that the sound waves uttered from the diaphragm are radiated through the circular opening. In such a case, the circular opening in the arrangement serves as a point sound source through which the sound waves from the diaphragm emerge out of the casing in generally spherical wavefronts. These sound waves with generally spherical wavefronts, like those peculiar to a point-sound-source loudspeaker such as a cone type dynamic loudspeaker, accompany sound pressures propagating at different velocities from those of the vibrating particles in the air, so that they sound somewhat unnatural to the human ear. On the other hand, the sound waves from the point sound source tend to rapidly diffuse and damper in the air, and thus the frequency band obtained is narrow.
In the prior art arrangements, the piezoelectric crystal diaphragm tends to vibrate in different phases at its different portions (split vibrations) and the resonance frequencies which cause such split vibrations are high, so that the fidelity of the reproduction is good only for high audio frequencies.